A plasma-enhanced chemical vapor deposition (PECVD) process is a process widely used in the manufacture of semiconductor devices for depositing layers of electronic materials on various substrates. In a PECVD process, a substrate is placed in a vacuum deposition chamber equipped with a pair of parallel plate electrodes or other means of coupling electrical energy into the chamber, such as a helical coil. The substrate is generally mounted on a susceptor which is also the lower electrode. A flow of a reactant gas is provided in the deposition chamber through a gas inlet manifold which also serves as the upper electrode. A radio frequency (RF) voltage is applied between the two electrodes which generates an RF power sufficient to cause the reactant gas to form a plasma. The plasma causes the reactant gas to vigorously react and deposit a layer of the desired material on the surface of the substrate body. Additional layers of other electronic materials can be deposited on the first layer by providing in the deposition chamber a flow of a reactant gas containing the material of the additional layer to be deposited. Each reactant gas is subjected to a plasma which results in the deposition of a layer of the desired material.
In recent years, large liquid crystal cells have been used for flat panel displays. These type of liquid crystal cells contain two glass plates joined together with a layer of a liquid crystal material sandwiched therein. The glass substrates have conductive films coated thereon with at least one of the substrates being transparent. The substrates can be connected to a power source to change the orientation of the liquid crystal material such that various areas of the liquid crystal cell can be accessed by proper patterning of the conductive films. More recently, thin film transistors (TFT) have been used to separately address areas of the liquid crystal cell at very fast rates. This type of liquid crystal cells are useful for active matrix displays such as TV and computer monitors.
As the requirements for resolution of liquid crystal monitors increase, it becomes desirable to separately address a plurality of areas of the liquid crystal cell, called pixels. In a modern display panel, more than 1,000,000 pixels are normally present and the same number of transistors must be formed on the glass plates such that each pixel can be separately addressed and latched into one of two stable states.
Two major types of thin film transistor devices that are in commercial usage are the back channel etched (BCE) thin film transistor and the etch stopped (E/S) thin film transistor. The efficiency of manufacturing such amorphous silicon-based thin film transistors has been limited by the PECVD process utilized. It is difficult to produce good quality gate silicon nitride (g-SiN.sub.x) and amorphous silicon (a-Si) films at high deposition rates to achieve manufacturing efficiency and high throughput. The term SiN.sub.x is used to represent all silicon nitrides that may or may not have the exact stoichiometric ratio of Si:N at 3:4, i.e., Si.sub.3 N.sub.4. For instance, SiN.sub.x includes all silicon nitrides that have an atomic ratio of Si:N that is higher or lower than 3:4. The term amorphous silicon indicates a silicon that has a completely amorphous structure without any crystallinity.
In a typical BCE TFT device, the deposition of thick films of g-SiN.sub.x and a-Si is necessary. The film thickness required is normally in the range between 250 to 500 nm. In order to produce good quality films, a typical deposition rate used in the industry is approximately 20 nm/min. At such a low deposition rate, the substrate manufacturing process is very inefficient. However, when a fast deposition rate such as 300 nm/min is used, films of unacceptable quality are produced.
It is therefore an object of the present invention to provide a method of depositing multiple layers of thick films of g-SiN.sub.x and a-Si on a thin film transistor substrate at high deposition rates such that it can be suitably used in a manufacturing process.
It is another object of the present invention to provide a method of depositing thick films of g-SiN.sub.x and a-Si on a thin film transistor substrate at high deposition rates while producing films of superior quality suitable for use in a manufacturing process.